To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Starting price: 350 euros.
2. Current condition: 75 (MBC), which is a good condition.
3. Auction history: The coin has been auctioned 4 times and has sold on all occasions, indicating a constant interest in the coin.
4. Previous hammer prices:
- In March 2022, it sold for 360 euros with a condition of 30.
- In January 2023, it sold for 320 euros with a condition of 70.
- In September 2024, it sold for 280 euros with a condition of 80.

Given that the current condition of the coin is 75, which is better than the conditions of the previous auctions (30 and 70), but slightly lower than the condition of 80, we can infer that the hammer price could be in a range above 320 euros, but perhaps not as high as 360 euros.

Considering the starting price of 350 euros and the history of hammer prices, an estimated maximum purchase price could be between 350 and 370 euros, given that the condition is good and there is a history of hammer prices that supports this estimate.

Therefore, an estimated maximum purchase price could be 360 euros.